Wet strength is a desirable attribute for many disposable paper products that are required to maintain their wet integrity for an extended period of time prior to and during their intended use. Such products include toilet tissue, diapers, personal care products, and pre-moistened articles such as baby wipes and household cleaning wipes.
Permanent wet strength, however, is often an unnecessary and undesirable characteristic in paper products. Many paper products are disposed of after brief periods of use into landfills, incinerators, etc. which is inconvenient and can pose a significant burden on the solid waste stream. It is therefore preferable in many cases to direct used paper products to municipal sewage treatment facilities or private septic systems. Unfortunately, this procedure is often not available if the product is “unflushable”. Clogging of sewage and septic systems can result if the product permanently retains hydrolysis-resistant strength properties. To address this problem, efforts have been undertaken to make binders that will provide paper products with sufficient “temporary” wet integrity in the presence of saline solutions, but minimal integrity when exposed to large amounts of waste water, such that they traverse plumbing and disintegrate in sewage and/or septic systems.
Specifically, it has been attempted to produce disposable fibrous products which maintain a relatively high wet strength in the presence of solutions with elevated ion concentrations, but become more dispersible when in contact with solutions having a lower ion concentration. These ion sensitive, water dispersible polymer formulations are well known in the art. There is disclosed for example, in U.S. Pat. No. 6,429,261 to Lang et al. a polymer formulation comprising a triggerable copolymer of acrylic acid, NaAMPS, butyl acrylate and 2-ethylhexyl acrylate as well as a non-crosslinkable co-binder emulsion polymer comprising poly(ethylene—vinyl acetate), wherein the polymer formulation is insoluble in a neutral salt solution containing at least about 1 weight percent salt and wherein the triggerable copolymer is soluble in water containing up to about 200 ppm of one or more multivalent ions. The polymers in Lang are typically prepared by solution polymerization.
U.S. Pat. No. 6,683,129 to Eknoian discloses salt sensitive binders in aqueous emulsions comprising methacrylic acid and acrylate monomers such as butyl acrylate and 2-ethylhexyl acrylate. The ion-sensitive binders are applied as emulsion compositions.
U.S. Pat. No. 6,291,372 of Mumick et al. discloses ion sensitive binders for fibrous material. A water soluble polymer binder is disclosed that includes about 25 to about 85 weight percent of unsaturated carboxylic acid ester terpolymer; as well as from about 5 weight percent to about 35 weight percent of a divalent ion inhibitor and from about 10 weight percent to about 60 weight percent of a hydrophilic cross-linkable emulsion polymer. The polymer binder is useful for binding absorbent webs of the class used in personal care products such as pre-moistened wipes.
U.S. Pat. No. 5,631,317 to Komatsu et al. discloses a process for producing a self-dispersing and salt-sensitive polymer. The formulations include a) from about 35 to about 65 weight percent of acrylic acid; b) from about 10 to about 45 weight percent of an acrylic ester such as 2-ethylhexyl(meth)acrylate or lauryl(meth)acrylate; and c) from about 20 to about 45 weight percent of a second acrylic ester such as ethyl(meth)acrylate, isopropyl(meth)acrylate, or n-butyl(meth)acrylate. The polymers in Komatsu are polymerized in a mixture of water and organic solvent, and the solvent is subsequently evaporated such that the binder is provided in an aqueous dispersion.
Still further references of interest with respect to salt sensitive binders include the following: U.S. Pat. No. 5,312,883 and U.S. Pat. No. 5,317,063 both to Komatsu et al. which disclose water soluble salt sensitive polymers; and U.S. Pat. Nos. 6,127,593 and 6,433,245, both to Bjorkquist et al. which disclose flushable fibrous structures.
Many of the references discussed above, such as Lang and Komatsu, produce ion sensitive binders by solution polymerization. Others, such as Eknoian, employ emulsion polymerization and provide the binders as emulsion compositions. These approaches are typical in salt-sensitive binder applications.
In contrast to the above procedures, emulsion polymers converted into water soluble polymers, whereby the emulsion polymer is put into solution by increasing the pH have not heretofore been used as salt-sensitive binders. Emulsion polymerized, alkali-solubilized polymers have been employed primarily in applications such as thickeners. Thickeners are added to aqueous systems to increase the viscosity to a desired level and are frequently added to materials such as paints, polishing and cleaning compositions, pharmaceuticals, among others. U.S. Pat. No. 5,073,591 to Tsaur discloses a method for producing an alkali soluble emulsion polymer for use as a thickener. The method comprises emulsion-polymerizing in an aqueous media a polymer having an acidic moiety and an amino moiety. The emulsion polymerization occurs in acidic media and then the pH of the emulsion is raised to dissolve the polymer. Tsaur notes that when the pH of the emulsion is increased, the dispersed composition dissolves, and the viscosity rapidly increases.
U.S. Pat. No. 4,384,096 to Sonnabend discloses a liquid emulsion copolymer containing a carboxylic acid monomer, a monovinyl ester monomer, and a nonionic vinyl surfactant ester monomer. The emulsions are stable as dispersions in solutions with a pH below 5.0, but dissolve as the pH is increased. The dissolved emulsions are said to be useful as thickeners in applications such as latex paint.
Other references of interest include U.S. Pat. No. 6,063,857 to Greenblatt et al. which discloses a hydrophobically-modified alkali soluble emulsion polymer which is neutralized to at least 60% of its acidic groups. The polymer contains as the hydrophobic monomer, a surfactant ester. The polymers in Greenblatt may be used as thickening agents in paints, adhesives, nonwovens, textiles, etc. Likewise, U.S. Patent Application Publication No. 2004/0151886 to Bobsein et al. discloses a composition for paper coatings which contain 1) a binder copolymer and 2) a hydrophobically modified alkali-soluble emulsion polymer. U.S. Pat. No. 4,801,671 to Shay et al. discloses an alkali soluble copolymer containing a monomer with surfactant characteristics and a monomer with carboxyl functionality. Here again, the copolymers are used as thickeners. While the substantial increase in viscosity associated with water-soluble emulsion polymers is beneficial in thickening applications, it is an undesirable property in the field of nonwoven binders because the viscosity must be kept within processable limits.
Emulsion polymerization offers several advantages in the production of nonwoven binders. For example, emulsion polymerization is cost effective and environmentally safe, allowing for the production of a high-solids composition without the necessity of removing unwanted solvent. However, the present Applicants have observed that emulsion binders may not “trigger” as well as solution binders. “Triggering” is a critical mechanism in salt-sensitive applications whereby the polymer becomes insoluble in concentrated salt solutions, yet remains dispersible when in contact with solutions containing a lower concentration of ions. Also, the film-forming mechanism in emulsion polymers is different and less predictable than solution polymers. On the other hand, solution polymerization is disadvantaged from a processing standpoint because the solvent must be removed and the resulting compositions typically do not achieve as high of a solids content as can be produced with emulsion polymerization.
Thus, despite the contributions in salt-sensitive binders and products incorporating them, there still exists a need for high quality salt-sensitive binders which may be produced safely and efficiently.